The use of a sterilant such as ozone or hydrogen peroxide is established for removing many bacteria and other pathogenic microorganisms from an enclosed area environment. Unfortunately, such sterilants are also toxic or hazardous to higher life forms such as humans or domesticated animals, and great care has to be taken to reduce the concentration of the sterilant to safe levels before allowing access.
Contaminants to be removed may be of biological or of synthetic origin, including bacterial, viral, and other pathogens, or synthetic toxic agents such as compounds that interfere with biological processes. The environments may include areas where plants are grown such as greenhouses, food processing areas such as kitchens, hotel rooms, conference centres, isolation rooms and other areas in hospitals etc that may have been contaminated as well as rooms in dwellings and ambulances etc, and places where animals are kept such as on farms and zoos, especially quarantine areas, but including henhouses or other areas of high concentrations of animals.
Decontamination equipment to be used may be fixed or portable with electrical power from the mains supply, or from internal rechargeable batteries that are periodically recharged.
For ease of description, the following description concentrates on the use of ozone as the sterilant.
It is generally known that it may be advantageous to humidify the atmosphere prior to or during sterilisation. Humidification of the air may be achieved by spaying a mist of water droplets from a suitable nozzle(s), or by passing steam into the environment that would normally be at a temperature between 10° C. and 45° C., before, at the same time, or subsequent to introducing ozone into the environment. It is believed that the higher the humidity the better—the water enhances the effectiveness of the ozone, perhaps through formation of hydroxyl radicals that are especially potent oxidants, but this may depend on the actual prevailing conditions. At present, it is envisaged that humidification to greater than about 70% relative humidity will be preferred, but other humidity levels may be used.
As indicated above, the ozone may react with water vapour to form hydroxyl radicals, a particularly powerful oxidant, and it is probable that it is hydroxyl radicals that are the highly active agent. Combination of hydroxyl radicals in three body reactions would lead to formation of hydrogen peroxide that also has powerful antiseptic properties.
Ozone may be produced from a suitable ozone generator such as irradiating oxygen with ultraviolet irradiation or electrical techniques such as those involving corona discharge or plasma formation. Preferably the source of oxygen contains only limited amounts of nitrogen (eg less than 15%) to minimise the formation of undesirable nitrogen oxides. Although air may be used as the source of ozone, it is preferred to use pure oxygen or oxygen-enriched air.
The amount of ozone in the environment should be maintained at a level and time sufficient to destroy all of the contaminants present. Typical values are at least 10-50 ppm ozone and preferably 20-40 ppm ozone for 20-120 minutes, and preferably 30-60 minutes.
After the ozone and its derivatives have decontaminated the environment, the atmosphere of the environment is suitably circulated using a fan through a filter to remove particulate matter that might be present, and then over a catalyst to convert remaining ozone and its derivatives, such as hydrogen peroxide, to oxygen, or oxygen and water, so that the environment is safe for human or animal occupation.
The procedure described above may be accomplished using a computer-controlled system—for maintaining humidification, ozone levels for predetermined periods, and thereafter switching these off, and generating a flow of the atmosphere through a special catalyst to reduce the ozone to a safe level. Sensors provide inputs for this control, and predictive computer models enable reliable estimates to be made of the times needed for decontamination and the times needed to reduce the ozone to safe levels.
The prior art indicates active ozone decomposition catalysts include formulations containing metal components such as platinum or oxides such as those of manganese and other transition metal elements. Surprisingly, we have found platinum catalysts do not work well in this application involving high ozone levels and high humidity levels at ambient temperatures. Moderately good initial performance was observed but deactivation was very rapid. This may be due to strong adsorption of water and/or oxygen species on the active sites, and highlights the fact that this ozone-destruction application is very demanding. Highly-loaded MnO2 catalysts had good initial activity, and while initially better than platinum, they quickly deactivated in use. We found removal of adsorbed species by vacuum oven treatment overnight at 150° C. restored a portion of the lost activity, but the combination of rapid deactivation and slow regeneration clearly means that this catalyst is certainly not a practical solution.
Some prior art suggests silver oxide/MnO2 catalysts can have very good ozone decomposition activity. Silver oxide/MnO2 catalysts were prepared and tested, but again they deactivated quickly under the unusual conditions of very high humidity and very high ozone concentrations at low temperature. It was thought humidity in particular was responsible for catalyst deactivation, and there was some experimental evidence to support this.